The use of multiple antennas at both the transmitter and receiver of a wireless communication system along with related baseband signal processing is referred to as Multiple-Input Multiple-Output (MIMO) technology. In a MU-MIMO system, precoding is applied at the transmitter in order to suppress mutual interference experienced by each receiving station caused by transmissions to other receiving stations. To those skilled in the art, MU-MIMO precoding, refers to spatial encoding of the transmitted signal based on propagation channel. In order to apply MU-MIMO precoding, the transmitting station is required to know the Channel State Information (CSI) of the radio channels connecting it to each of the receiving stations for transmission. In 3GPP LTE systems, it is common for the receiving stations (e.g., user equipments) to measure CSI and report CSI to the transmitting station (e.g., eNodeB) via an uplink feedback channel. The content of CSI feedback contains RI (rank indicator), CQI (channel quality indicator), and PMI (precoding matrix indicator) for each downlink component carrier (CC).
MIMO transmission technique is essential to achieve higher data throughput utilizing spatial characteristics of the wireless channel. For example, by utilizing spatial multiplexing techniques, multiple streams of signals can be transmitted over multiple layers from a base station to user equipments (UEs) achieving higher data throughput. In general, this can be achieved with some knowledge of the downlink channel, which can be obtained from UE feedback or estimated by the base station via channel reciprocity. Taking downlink FDD LTE for example, the UE feedback CSI information contains RI, CQI, and PMI. Other wireless communication standards such as Wi-Fi also define similar mechanism for spatial multiplexing.
Currently, most hand-held mobile devices are expected to have two receive antennas, and most receiving and transmitting techniques are developed with this expectation. Although the base station can be equipped with more antennas than the mobile device, the maximum rank of the overall wireless channel is limited by the minimum antenna configuration on either side. As a result, the maximum rank of the wireless channel is two. The rank determines the maximum number of spatial layers that can be transmitted without severe cross interference between the layers, under ideal conditions. For example, the base station can transmit two streams simultaneously over two layers if the rank is two. This case is usually expected to happen when the mobile device enjoys high signal quality, e.g., when it communicates with a very close base station in a small cell.
To realize spatial multiplexing in wireless communication systems, a mobile device reports its preferred number of spatial transmission layers, i.e. its rank to its serving base station. The periodicity of this operation can be configured differently. Under the assumption of two receive antennas, the maximum possible rank of the wireless channel is two. The base station usually adopts the recommendation in the UE report in its downlink scheduling.
A solution for transmitting higher number of layers in the downlink than the number of receive antennas, the rank of the wireless channel, or the recommended value from the UE is sought.